The Arabidopsis homolog of microspherule protein 1 is essential for embryogenesis and interacts with the Myb-like transcription factor DRMY1.

The evolutionarily conserved microspherule protein 1 (MCRS1) has diverse functions, ranging from transcriptional regulation to stabilization of microtubule minus ends in acentrosomal spindles in mammals. A previous study suggested that in the model plant Arabidopsis thaliana, inactivation of an MCRS1 homolog gene led to aborted embryogenesis. To test whether this lethality was caused solely by sporophytic defects, we used the heterozygous emb1967-1/mcrs1-1 mutant for reciprocal crosses with the wild-type plant and found that the MCRS1 gene was dispensable for the development of both male and female gametophytes. An MCRS1-GFP fusion protein was expressed in the mcrs1 mutant and suppressed the mutation as evidenced by restored growth. This functional fusion protein exclusively localized to interphase nuclei and became unnoticeable during mitosis before reappearing in the reforming daughter nuclei. Affinity purification of the MCRS1-GFP protein specifically recovered the Myb-like transcription factor DRMY1 (Development Regulated Myb-like 1) but not microtubule-associated factors. Direct MCRS1-DRMY1 interaction was also demonstrated by a localization-based assay in living cells. Thus, we hypothesized that MCRS1's function was perhaps linked to transcription factors like DRMY1 and its paralog DP1 for regulation of gene expression during sporophyte development.